Ed Husain, Senior Fellow for Middle Eastern Studies
Yes and no. The armed uprising against Syrian President Bashar al-Assad began in March 2011 and the conflict has claimed over 70,000 lives. The presence of Sunni Salafi fighters in the Syrian opposition affects Iran because their proliferation and prominence are at odds with the Iranian government's form of Shiite Islam.
Most Salafis consider Shiite Muslims as heretics, and the fight against Assad is colored for many Sunni Muslims by a sectarian outlook; Assad's regime is dominated by Alawites, a minority sect within Shiite Islam, ruling over a Sunni majority population.
Many of the Sunni radical fighters are supported financially by Turkey, Qatar, Kuwait, and Saudi Arabia, and rhetorically by Egypt, Tunisia, Jordan and others. These Sunni-majority nations are not part of the Iranian axis in the Middle East, and the Gulf countries that are supporting Salafi fighters inside Syria do so to damage Iran's outreach and interests in the region.
So yes, Iran's influence becomes weakened at first sight if the Syrian opposition wins. But the Iranian regime can (and has) created common cause with Sunni radicals in the recent past. Sunni Hamas was openly in the pay of the Iranian clerics. And Al-Qaeda operatives enjoy immunity inside Iran.
Very few people foresaw that the Iraq the United States helped to liberate from Saddam Hussein would become an ally of Tehran, as it is today. Similarly, conventional wisdom suggests that Iran's long-term interests will be weakened by the fall of Assad, but one cannot rule out greater Iranian involvement through militias and proxies from Iraq, Lebanon, and Syria's Alawite and Shiite populations.